In recent years, in a thin film transistor used as a switching element of a display device such as an active matrix type liquid crystal display element or an organic electroluminescent element, the next generation thin film transistor has been actively developed. That is, the development of a thin film transistor using a transparent conductive oxide poly-crystal thin film mainly made of ZnO as a channel layer. This thin film is highly-mobile than amorphous silicon, and can be deposited at low temperature, and can form a flexible transparent thin film transistor on a substrate such as plastic plate and film. Further, since it is transparent for visible light, a shielding layer and the like are not required.
An example of the manufacturing method of the thin film transistor mainly made of ZnO as described above will be described below. A semiconductor layer including an intrinsic zinc oxide is formed on a gate insulation film, and a protection film having the same shape as an end face of the semiconductor layer is formed, and on the upper surface thereof, an upper layer insulation film is formed, and after that, a contact hole is formed. The semiconductor layer upper surface exposed through this contact hole is formed with an ohmic contact layer by an n-type ZnO or a region in which a semiconductor layer and an electrode are in contact with each other is subjected to low-resistance processing, and after that, on that upper face, a source electrode and a drain electrode are formed (US Patent Application Publication No. 2006/043447A1). However, in the above described thin film transistor, it is difficult to form a thin film which is flat and high in uniformity in the poly-crystal thin film. Consequently, a fluctuation of the quality numerously arises.
Hence, an attempt is made to develop a thin film transistor which uses a transparent amorphous oxide semiconductor film for the channel layer. In the amorphous thin film, it is possible to form a thin film flat and high in uniformity. In “Applied Physics Letters, 89, 112123, (2006)”, it is disclosed that a transparent amorphous oxide semiconductor film (a-IGZO) is deposited at the room temperature by a magnetron sputtering method as a channel layer of the thin film transistor, thereby to obtain a semiconductor layer good in flatness and uniformity. This thin film transistor has electric field-effect mobility of 12 cm2V−1s−1, and is a good thin film transistor.